linux-kernel - Re: RFC [PATCH v4 2/7] Enable balloon drivers to report inflated memory

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Message-ID: <72e535ce-80eb-a02f-970c-6a9c80da0a24@redhat.com>
Date:   Fri, 14 Oct 2022 15:01:08 +0200
From:   David Hildenbrand <david@...hat.com>
To:     Alexander Atanasov <alexander.atanasov@...tuozzo.com>,
        Nadav Amit <nadav.amit@...il.com>
Cc:     "Michael S. Tsirkin" <mst@...hat.com>,
        Andrew Morton <akpm@...ux-foundation.org>, kernel@...nvz.org,
        Linux Virtualization <virtualization@...ts.linux-foundation.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Linux MM <linux-mm@...ck.org>
Subject: Re: RFC [PATCH v4 2/7] Enable balloon drivers to report inflated
 memory

On 14.10.22 14:50, Alexander Atanasov wrote:
> Hello,
> 
> On 11.10.22 12:23, David Hildenbrand wrote:
>>>>>> Sounds to me that all you want is some notifier to be called from
>>>>>> adjust_managed_page_count(). What am I missing?
>>>>>
>>>>> Notifier will act as an accumulator to report size of change and it
>>>>> will make things easier for the drivers and users wrt locking.
>>>>> Notifier is similar to the memory hotplug notifier.
>>>>
>>>> Overall, I am not convinced that there is any value of separating the
>>>> value
>>>> and the notifier. You can batch both or not batch both. In addition,
>>>> as I
>>>> mentioned, having two values seems racy.
>>>
>>> I have identified two users so far above - may be more to come.
>>> One type needs the value to adjust. Also having the value is necessary
>>> to report it to users and oom. There are options with callbacks and so
>>> on but it will complicate things with no real gain. You are right about
>>> the atomicity but i guess if that's a problem for some user it could
>>> find a way to ensure it. i am yet to find such place.
>>>
>>
>> I haven't followed the whole discussion, but I just wanted to raise that
>> having a generic mechanism to notify on such changes could be valuable.
>>
>> For example, virtio-mem also uses adjust_managed_page_count() and might
>> sometimes not trigger memory hotplug notifiers when adding more memory
>> (essentially, when it fake-adds memory part of an already added Linux
>> memory block).
>>
>> What might make sense is schedule some kind of deferred notification on
>> adjust_managed_page_count() changes. This way, we could notify without
>> caring about locking and would naturally batch notifications.
>>
>> adjust_managed_page_count() users would not require changes.
> 
> Making it deferred will bring issues for both the users of the
> adjust_managed_page_count and the receivers of the notification -
> locking as first. And it is hard to know when the adjustment will
> finish, some of the drivers wait and retry in blocks. It will bring
> complexity and it will not be possible to convert users in small steps.

What exactly is the issue about handling that deferred? Who needs an 
immediate, 100% precise notification?

Locking from a separate workqueue shouldn't be too hard, or what am i 
missing?

> 
> Other problem is that there are drivers that do not use
> adjust_managed_page_count().

Which ones? Do we care?

-- 
Thanks,

David / dhildenb